Non-oxygenated blood enters the right atrium via the inferior and superior vena cava.

Increase level of blood in the right atrium causes the tricuspid valve to open and drain the blood to the right ventricle.

Pressure of blood in the right ventricle causes the pulmonic valve to open and non-oxygenated blood is directed to the pulmonary artery then to the lungs.

Exchange of gases occurs in the lungs. Highly oxygenated blood is returned to the heart via the pulmonary vein to the left atrium.

From the left atrium the pressure of the oxygenated blood causes the mitral valve to open and drain the oxygenated blood to the left ventricle.

Left ventricle then pumps the oxygenated blood that opens the aortic valve. Blood is then directed to the ascending and descending aorta to be distributed in the systemic circulation.

Fetal Circulation Sequence

Exchange of gases occurs in the placenta. Oxygenated blood is carried by the umbilical vein towards the fetal heart.

The ductus venosus directs part of the blood flow from the umbilical vein away from the fetal liver (filtration of the blood by the liver is unnecessary during the fetal life) and directly to the inferior vena cava.

Blood from the ductus venosus enters to the inferior vena cava. Increase levels of oxygenated blood flows into the right atrium.

In adults, the increase pressure of the right atrium causes the tricuspid valve to open thus, draining the blood into the right ventricle. However, in fetal circulation most of the blood in the right atrium is directed by the foramen ovale (opening between the two atria) to the left atrium.

The blood then flows to the left atrium to the left ventricle going to the aorta. Majority of the blood in the ascending aorta goes to the brain, heart, head and upper body.

The portion of the blood that drained into the right ventricle passes to the pulmonary artery.

As blood enters the pulmonary artery (carries blood to the lungs), an opening called ductus arteriosus connects the pulmonary artery and the descending aorta. Hence, most of the blood will bypass the non-functioning fetal lungs and will be distributed to the different parts of the body. A small portion of the oxygenated blood that enters the lungs remains there for fetal lung maturity.

The umbilical arteries then carry the non-oxygenated blood away from the heart to the placenta for oxygenation.